Affiliate referrals not registering from private forum

Hi - there is a private support forum at The referrals from there come through on Google Analytics as

Stefan from Datafeedr signed up to my Affiliate program using the Affiliates plugin and has entered his domain as but clicks from their support forum to my site are not registering as affiliate clicks.

I have tested this separately with my own clicks and they work elsewhere so I'm presuming it's because their forum is private? It's weird because Analytics spots the correct referrer but your Affiliates plugin does not.

Can you help me fix this please?